Messages in this thread Patch in this message | | | Date | Thu, 22 Nov 2007 19:52:38 +0100 | From | Pavel Machek <> | Subject | Re: nohz and strange sleep latencies |
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Hi!
> > > to me this has the feeling of lapic breakage in C2 mode. Does it get any > > > better if you boot with 'nolapic'? (but that might in turn turn off > > > high-res timers and nohz in essence) Thomas, any ideas? > > > > Hmm, lapic is considered unstable in c2 by default. You have to tell > > the kernel that you trust it in C2 on the command line. > > yeah, i was wondering about that too. ACPI enumerated them properly at a > certain stage: > > ACPI: CPU0 (power states: C1[C1] C2[C2] C3[C3]) > ACPI: CPU1 (power states: C1[C1] C2[C2] C3[C3]) > > but perhaps somehow we miss this fact and fail to turn off the lapic > clockevents drivers?
Ok, I guess I'm lost. If I offline second CPU, I immediately get 1000Hz timer tick... is that expected?
I'm trying to decide when system is idle (lets say that means "no user task is scheduled to wakeup within 10 seconds)... I added some instrumentation to nohz subsystem, but it does not behave like I'd expect: even if I run "while true; do sleep .01; done" loop, I see nohz preparing for 5 seconds sleep... while it seems obvious that it can only be 10msec sleep, and with max_cstate=1, it works that way... Plus, nte->start_pid seems to contain some random numbers :-(.
What am I doing wrong?
(Patch for illustration, I can generate full diff against vanilla, but...) Pavel
+++ b/kernel/time/tick-sched.c @@ -229,11 +232,13 @@ void tick_nohz_stop_sched_tick(void) if (delta_jiffies > 1) cpu_set(cpu, nohz_cpu_mask); + { + int user_wait = get_next_timer_interrupt(last_jiffies, 1) - last_jiffies; + + if ((user_wait > HZ/10) && (num_online_cpus() == 1)) + printk("NOHZ: user ready for %d:%d sec wait (kernel %d:%d sec wait), naughty %d\n", user_wait/HZ, user_wait%HZ, delta_jiffies/HZ, delta_jiffies%HZ, naughty_pid); + } + /* * nohz_stop_sched_tick can be called several times before * the nohz_restart_sched_tick is called. This happens when +++ b/kernel/timer.c @@ -691,6 +693,12 @@ static unsigned long __next_timer_interr if (tbase_get_deferrable(nte->base)) continue; + if (flags && (nte->start_pid < 1)) + continue; + + if (flags) + naughty_pid = nte->start_pid; + found = 1; expires = nte->expires; /* Look at the cascade bucket(s)? */
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